An inhibitor of this type is known from South African Pat. No. 721,862, which discloses such a copolymer and a process for preparing same, and which also proposes to dissolve this copolymer in petroleum to prevent crystallization of paraffin. According to said patent there is preferably used a copolymer of 4-vinyl pyridine with an acrylate of a saturated, straight-chained alcohol having at least 14 carbon atoms, in particular 18-20 carbon atoms, or a mixture of such alcohols. The molecular weight of the copolymer is preferably not higher than 100,000; and a molecular weight of about 40,000, in an amount of up to 2 wt.%, preferably 0.002-0.2 wt.%, has appeared to be highly suitable.
The molar ratio of vinyl pyridine and acrylate ester in the polymer chain may vary between 5:1 and 1:20, and preferably ranges between 1:1, and 1:10. According to said patent, the copolymer may suitably be added to the oil in the form of a solution in a hydrocarbon, such as toluene.
When an oil contains paraffin, this crystallizes out on cooling and in many cases there is formed a three-dimensional network of scales and/or fine needles, which is filled with oil. Such a structure does not flow or hardly so and it is very difficult to pump, while in pipe lines and in storage tanks or in ship tanks mostly considerable amounts of oil remain behind between the paraffin crystallized onto the walls.
When, however, the above-mentioned copolymer is added to the oil in still hot condition, i.e. when the paraffin is still in solution, the crystallization of the paraffin proceeds differently, namely with formation of more massive crystals, so that the temperature at which the oil no longer flows is lowered considerably and, therefore, the above problems occur to a far less extent.
As long as a paraffin containing crude oil is present in an oil containing layer, it usually has an elevated temperature which varies according to the depth, but usually the paraffin is then still fluid. When this oil is pumped up through a borehole, it passes the layers thereabove, the temperature of which becomes gradually lower, so that also during pumping up the oil is cooled and paraffin may crystallize onto the walls of the pump tube or of the above-ground pipes as soon as the saturation temperature is transgressed. The crystals so formed gradually decrease the free diameter of the tube, and the recovery of oil from such boreholes is difficult in such cases, because the pump tube has to be cleaned periodically, for example, by pumping through hot water or by introducing a scraper therein.
In the said South African patent it is therefore proposed to pump the above copolymer in the form of a solution in e.g. toluene through a narrow tube to the bottom of the borehole, where this solution is then mixed with the petroleum and impedes the crystallization of the paraffin, so that during pumping up the viscosity increases to a far less extent.
However, with such methods there occur considerable difficulties.
The copolymer itself is a solid substance at all the temperatures to be considered, while a solution in e.g. toluene at a concentration of e.g. 40% is also solid at 20.degree. C. and therefore has to be heated to about 40.degree. C. before it can be added to the oil.
Of course, this addition is effected most favorably when all the paraffin is still in solution, that is at an elevated temperature, but it is inconvenient when also the inhibitor to be added has to be heated prior to being added.
These drawbacks are particularly troublesome when the inhibitor has to be added in the oil recovery field, either at the bottom of the borehole or in a pump line, and over long periods, e.g. some weeks or months. For in the oil recovery field it is very troublesome in practice to keep the dosing apparatus continuously at a sufficiently high temperature to prevent solidification of the composition and hence blocking of the supply line with certainty. Yet this is necessary in most cases, since the dosing apparatus has to work completely automatically and without supervision for a long time and preferably should be as simple as possible.
It appeared not to be possible to avoid this high solidification point of the solution in toluene by using another oil-miscible solvent, because also in that case the solution appears to have a solidification point or saturation point well above room temperature, while from a technical viewpoint it is desirable that the dosing can normally be continued, even at ambient temperatues as low as -15.degree. C. The use of a more diluted solution is not suitable either, since in that case extremely diluted solutions (e.g. 1% or less) would have to be used, which on the one hand increases the cost and on the other hand would require a large-sized container for the solution. Since such diluted solutions are not suitable to be stored, it would furthermore be necessary to prepare same on the spot, which means an additional complication.
In view of the weather conditions prevailing in oil recovery fields, there is need for a composition which contains the above-mentioned copolymers in a not too low concentration (e.g. at least 7-8 wt.%), in the entire temperature range of from -15.degree. C. to +50.degree. C. is fluid and stable, and has a viscosity of at most 100 cP at 20.degree. C. and at most 300 cP at 5.degree. C.